The present invention relates to a method for fabricating a semiconductor device, and more particularly, to a method for forming a metal bit line in a flash memory device using a damascene process.
A bar resistance that is appropriate for an operation characteristic of a flash memory device may be difficult to obtain if tungsten (W) is used as a bit line when fabricating a flash memory device smaller than 60 nm. A method for forming a metal line using aluminum (Al) or copper (Cu) instead of tungsten has been introduced to address this problem. Typically, a method for forming a metal line using aluminum includes depositing aluminum and then performing a reactive ion etch (RIE) process. However, it may be difficult to obtain a uniformly etched surface. Furthermore, a loss of aluminum may occur on a bottom interface when forming the metal line using the RIE process. Thus, the aluminum line may be difficult to embody when a critical dimension (CD) of the metal line is low. A metal line including copper is formed using a damascene process. The damascene process generally has a better electro-migration characteristic than the RIE process.
The damascene process is commonly used to form a metal line including copper or aluminum. A barrier metal layer is provided on an interface of an insulation layer to decrease diffusion of the copper or aluminum into the insulation layer. When compared to the metal line including aluminum (hereinafter referred to as the aluminum line), the formation of the metal line including copper (hereinafter referred to as the copper line) using the damascene process provides a better electro-migration characteristic and better stability when performing a chemical mechanical polishing (CMP) process to isolate lines. However, the formation of the copper line generally requires an independent work space and apparatus for forming the copper line because of a diffusion characteristic of copper. It should be noted that copper has a fast diffusion speed in silicon or an oxide-based material used as an insulation layer.
In contrast, the aluminum line formed using the damascene process may provide advantages because the aluminum line may form a denser layer than the copper line. Also, aluminum may not diffuse into silicon or an insulation layer. However, the aluminum line is less stable than the copper line and therefore has a diminished electro-migration characteristic. Thus, the aluminum line may easily corrode. In particular, galvanic corrosion supplying electrons to a barrier metal layer may occur at a contact region with the barrier metal layer which includes a metal other than aluminum. Such a characteristic may increase the bar resistance of the metal line and adversely impact the reliability of the device.
After performing the CMP process for electrically isolating adjacent aluminum lines, a cleaning process is performed to remove slurry residue and polish by-products generated during the CMP process. The cleaning process typically uses an ammonia (NH3)-based or hydrogen fluoride (HF)-based cleaning solution. Consequently, the cleaning solution chemically damages the aluminum line and galvanic corrosion occurs. Therefore, an improved cleaning solution for decreasing the galvanic corrosion during a cleaning process is desired. FIG. 1 illustrates micrographic views of galvanic corrosion (referred to as ‘C’) generated between an aluminum line and a barrier metal layer during a cleaning process after a CMP process is performed.